1. Field of the Invention
The present invention relates to a method of manufacturing a steering wheel, and more particularly relates to a method of manufacturing a steering wheel, in which a coating layer is molded of a synthetic resin on a steering wheel core including an annular portion having a groove extending in the circumferential direction of the portion.
2. Description of the Prior Art
The cross section of at least the annular portion of the core of a steering wheel is U-shaped in order to reduce the weight of the core, namely, the weight of the wheel as a whole, as disclosed in the Japan Utility Model Application (OPI) No. 16559/87 (the term "OPI" as used herein means an "unexamined published application"). When such a steering wheel is to be manufactured, the core thereof is set in a molding cavity 54 defined by an upper molding die and a lower molding die 101 and 102, so that the outermost part 53 of the groove 52 of the annular portion 51 of the core is located as the top of the groove, as shown in FIG. 9. A synthetic resin 56 such as urethane, polypropylene and vinyl chloride is then injected into the cavity 54 through a gate 55 provided at the separation surfaces 112 of a molding die unit 100 consisting of the molding dies 101 and 102, so that a coating layer is molded of the resin on the core.
However, in such a method of manufacturing the steering wheel, if the injection of the synthetic resin 56 is completed in a short time, the synthetic resin injected through the gate 55 runs onto a wall 58 facing the gate and included in the annular portion 51 including another wall 57 and having the groove 52 defined between the walls 57 and 58 and gravitates on the wall 58 so that the resin starts filling the cavity 54 from the bottom thereof, the surface of the resin rises over the wall, and the resin flows down into the groove. In that case, since the resin 56 flows down into the groove 52 while involving much air, the coating layer is likely to have a cavity therein. Besides, the speed of the filling of the cavity 54 with the resin from the bottom of the cavity changes at the time of the inflow of the resin into the groove 52 so that the outside surface of the coating layer is likely to have a luster nonuniformity. Since the coating layer is likely to have such molding defects, it is difficult to properly manufacture the steering wheel in a short time. This is a problem.